Data recording disk drives are useful in the storage of large amounts of data for computer systems. The data is stored as a series of magnetic field transitions on a magnetic recording surface. The transitions are placed on the surface by a magnetic transducer commonly referred to as a magnetic recording head. The transducer converts electrical energy into a magnetic field, the polarity of which is switched according to the information to be recorded. The magnetic field causes magnetization to remain in the media after the field is removed. The data is stored as binary information in the polarity reversals, or transitions, remaining in the media. The transducer used with magnetic media may also act as a detector to detect data stored as magnetic transitions. The transducer senses a magnetic field emanating from the magnetized media. The sensed magnetic field is converted into an electric signal which varies depending on the polarity of the magnetic field. Data is then decoded from the electrical signal. When the transducer places data on the recording media, the transducer is said to have written data to the media. When the transducer detects previously written data on the media, the transducer is said to have read data from the media. In general, systems for storing and retrieving data to/from magnetic media may employ a single transducer to both read and write data, or they may employ dual transducers, one to read and one to write.
The recording media is in the form of a disk, typically with data being recorded on both surfaces. Multiple disks may be provided to increase the aggregate storage capacity of the disk drive. The center hole in the media is typically called a hub. The hub is the means by which the recording media attaches to a motor, through a spindle shaft, which rotates the recording media. The head is flown over the surface of the recording media by virtue of the air movement created when the disk rotates. The flying height must be large enough to minimize the probability of head and disk contacts that could be detrimental to data integrity, but small enough so that the magnetic field generated by the write transducer establishes magnetic transitions in the recording media surface and so that a magnetic field in the media can be sensed by the transducer.
The head is placed in proximity to the recording surface and positioned over the desired data track by an actuator arm, to which it is attached via a suspension. The actuator arm moves the head radially with respect to the media surface from a position near the hub (the inside diameter (ID)) to a position near the rim (the outside diameter (OD)). Data is commonly written onto the media surface between the ID and OD in the form of sequential concentric tracks. The track width is usually slightly larger than the width of the write transducer. The concentric tracks may be subdivided into one or more sectors.
The head must be accurately positioned over the desired data track to read or write data. Head positioning is typically accomplished by way of an actuator positioning servo controller driving a voice coil motor (VCM) attached to the actuator arm. The actuator positioning servo controller makes use of pre-recorded head positioning information as well as track and sector identification information to move the head from one track to another, known as seeking to a desired track, and to position the head over the center of the desired track and at the appropriate sector along the track. The positioning and identification information is pre-recorded on one or more of the disk surfaces, and consists of magnetic patterns which vary in both the radial and circumferential direction to provide the actuator positioning servo controller with feedback indicating the current position of the head relative to the desired track and sector.
Depending on the track positioning architecture of the disk drive, the track positioning information may be pre-recorded on a single surface of a multiple surface disk drive, usually referred to as dedicated surface servo, or in multiple radial wedges on each of the disk surfaces, usually referred to as embedded sector servo. A disk drive using dedicated surface servo positions the actuator using the information pre-recorded on the servo surface; the position of the data heads is mechanically slaved by the actuator to the servo head position. A disk drive using embedded sector servo positions the actuator using the information pre-recorded on the particular data surface being read by the data head. In some disk drives a combination of the two architectures is used. The embedded sector servo architecture is preferred for disk drives having higher track pitch since it is less sensitive to mechanical and thermal disturbances that affect the positioning accuracy of the data head.
The data head must be accurately positioned over the desired data track and data sector before user data can be stored or retrieved from a disk drive. The actuator positioning system accomplishes this by reading the pre-written positioning and identification information and using it to update the position of the actuator. The positioning and identification information in encoded onto the disk surface in the form of a pattern of accurately sized and spaced magnetic transitions, known as servo patterns, precisely recorded in both the radial and circumferential directions. To enable the required head positioning accuracy for data read and write operations, these servo patterns must be written with an accuracy in the radial direction such that the decoded radial position can be determined to within a small fraction of the data track width.
They also must be written with an accuracy in the circumferential direction such that the track to track relative circumferential positioning of the servo pattern is kept suitably small; in the most demanding servo encoding methods, this may require that the track to track relative circumferential positioning of the individual magnetic transitions is kept to a small fraction of the recorded transition spacing in the circumferential direction.
These precisely positioned servo patterns may be recorded onto the disk surfaces prior to assembling the disks into the disk drive or after the disks are assembled into the disk drive using a process referred to as servo-writing. In either case, the required radial positioning accuracy during servo-write is typically obtained using an external, laser interferometer-controlled rotary or linear actuator mechanically coupled to the drive. The required circumferential positioning accuracy and repeatability track-to-track is obtained using a clock head positioned over the disk surface. The clock head reads approximately equally spaced transitions pre-written on a track on the disk surface. The timing jitter in the clock head readback signal is typically reduced by processing the signal using a narrow bandwidth phase lock loop.
Once the proper clock track information has been established, the servo-write process consists of positioning the external actuator arm at successive radial locations and writing the appropriate magnetic transitions at multiple positions in the circumferential direction. The process is extremely sensitive to vibration, so a large, expensive granite table must be used to steady the drive. The process is also extremely sensitive to ambient temperature variations due to the large size of the electromechanical system used to position the actuator. To minimize this disturbance the process is usually performed in a temperature controlled room. To provide for the mechanical coupling to the actuator and to insert the additional clock head into the drive, the drive must remain open (or provide for the required openings) and unsealed during the process, necessitating the use of a clean room environment. Also, the final assembly procedure which occurs after the servowriting process may introduce stress to the drive's base plate when the cover plate is attached, causing tilting of the spindle shaft and actuator pivot axis and generating misalignment between the servo patterns on different disks. Thus, for all the above reasons, the servo-write process is both costly and error prone.
Recent developments in servo-writing have addressed some of the above-described problems. A semiconductor laser rotary encoder is used to generate position reference information. The rotary encoder connects to the actuator arm via an exposed pivot outside the disk drive. A mechanical coupling is used between the rotary encoder and the pivot to insure the integrity of the connection. A reference clock is generated using a patterned disk pasted onto an exposed portion of the spindle shaft which extends outside the disk drive. The patterned disk has light and dark sectors that reflect incident light with different intensity. The detected intensity pattern is used to generate the clock signal.
While the rotary encoder servo-write system eliminates the need for a clean room and a granite table, it incurs several drawbacks of its own. First, the mechanical coupling required between the semiconductor laser rotary encoder and the actuator arm adds to the cost of the disk drive and the complexity of the servo-write process, and limits the accuracy achievable. Second, to expose the spindle shaft for generating the reference clock, a double sealed bearing is required, again increasing the cost of the disk drive. Third, the reference clock generated by the patterned disk is not accurate enough for use in a high density disk drive.
Thus, there has heretofore existed an unmet need in the art for a servo-writing system that is non-invasive, requires no mechanical coupling to the actuator or the rotating disk spindle, and provides sufficient performance for use in modern, high density data recording disk drives. The present invention is directed to meeting this need.